DPWES Letters to Industry

Information Required on Infill Lot Grading Plans to Assist in the
Assessment of Adequate Drainage

Issue: Additional guidance on data to be provided
on infill lot grading plans to facilitate review of the adequacy of storm
drainage for compliance with Section 2-602 of the Zoning Ordinance and
Section 6-0203 of the PFM.

Background: At the request of the Board of
Supervisors, this agency created a committee to develop recommendations
for enhancing the County’s erosion and sediment control program and
review of adequate storm drainage to further minimize the potential for
impacts on adjoining properties. The committee, comprised of
both public and private sector representatives, recommended that
additional guidance be provided to submitting professionals regarding the
type of data about adjacent properties that must be provided with the
submission of infill lot grading plans. This recommendation was
referred to the Engineers and Surveyors Institute’s Fairfax Committee for
implementation. The Fairfax Committee’s membership includes both
public and private sector engineers. The guidance provided in this
letter is the result of the efforts of that committee.

Procedures: Submitting professionals are
reminded that infill lot grading plans must include sufficient
information in accordance with Section 6-0203 of the PFM to verify the adequate drainage
of surface waters as required by Section 2-602 of the Zoning Ordinance
and the Minimum Standards and Specifications of the Virginia Erosion and
Sediment Control Handbook. Many infill plans that are submitted for
review do not contain sufficient information about adjoining properties
to validate the adequacy of downstream systems. The information
identified below is considered to be the minimum necessary to allow
validation of adequate drainage and shall be included on all future
submissions of infill lot grading plans. Information in addition to
that identified below may be required during plan review as determined
necessary to validate the adequacy of downstream systems both during and
after construction.

Five Hundred (500) scale topographical map of the site and adjoining
properties with drainage divides depicted.

Outfall narrative in accordance with the requirements of Section 6-0200
of the PFM that addresses both
concentrated and sheet flow discharges from the site (see Letter to
Industry 03-10 for additional guidance on the preparation of outfall
narratives).

Use and zoning district of properties contiguous to the proposed
development.

Sufficient topography for the proposed development site as well as
sufficient topography and information about existing structures (e.g.
houses, sheds, and swimming pools) on downstream contiguous lots to
show on and off-site surface drainage to the point where an adequate
outfall is reached. (Note: field run or aerial topography
is required; however, alternative sources may be accepted as determined
by the Director if it is shown that access to adjoining properties has
been denied.)

Inverts of all existing and proposed culverts on the subject lot and
contiguous lots.

Flow arrows showing the direction of flow.

Appropriate runoff “c” factor for the design of erosion and sediment
controls (e.g. use of a 0.6 factor pursuant to Section 6-0202.14 of the
Public Facilities Manual).

If you have any questions or need additional information, please call
Michelle Brickner at 703-324-1780, TTY 711.